The present invention relates generally to diagnostic imaging and, more particularly, to a detector assembly for radiographic imaging having photodiodes with multiple charge storage devices. The multiple charge storage devices not only reduce the likelihood of detector saturation, but also support intraview x-ray source wobble.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Each detector element is characterized by a scintillator cell that releases light in response to the reception of x-rays. The light is collected by a photodiode, which provides an electrical signal output that is indicative of the x-rays impinged by the scintillator cell. Charge generated from the sensed light is then transmitted to a data acquisition system (DAS), either directly, or through a FET switch array. As CT detectors have become larger in z-coverage and finer pitch to improve resolution, a much larger number of interconnects are needed to connect the increasing number of photodiodes to the DAS. The most commonly practiced CT detectors include thousands of detector elements and, as a result, a comparable number of connections to a comparable number of detector elements share a common DAS input channel. Other known detectors provide charge storage at each pixel and share a single amplifier among many pixels. For example, one known detector utilizes a single DAS channel for each photodiode pixel. Other known detectors have been constructed such that as many as eight pixels connected in parallel to a single DAS channel. Connecting the photodiode pixels in parallel, however, degrades the resolution and is thus undesirable. In another known detector design, storage capacitors and FET switches are provided at each pixel. This construction is often referred to as a storage diode design. With such a design, the storage capacitor is often provided by the native capacitance of the photodiode itself. While such a construction does allow for one amplifier to be shared among many pixels, and each pixel is read individually so as not to degrade resolution, it does not permit so-called source “wobble”. Source wobble requires that that the integrated signal at each pixel be sampled over the same time interval. This is not possible with the conventional storage diode detector design.
Therefore, it would be desirable to have a CT detector with a multi-charge storage device pixels as well as a CT detector whose pixels are connected in a way that multiple pixels can share a readout channel as well as support source wobble